Particularly in motor vehicles, but also in other areas, mechanical relays are increasingly being replaced by semiconductor switches such as, for example, transistors. This involves the application primarily of semiconductor switches that are additionally equipped with a logic that makes it possible to diagnose undesirable operating states such as, for example, short circuit, overload, overtemperature and/or open-load. One conventional application of such “intelligent” electronic switches is, e.g., modules for automobile lighting systems, wherein, for example, incandescent and similar lamps represent a problematic load for electronic switches.
In order firstly to minimize the losses in the conducting state of the electronic switches and secondly to prevent the area of the transistor from increasing significantly, the area-specific resistances of the transistors are minimized. At the present time they are for example already at values of between 150 mohm·mm2 to 75 mohm·mm2 at 150°. On account of the PTC (positive temperature coefficient) behavior of the incandescent lamps, high switch-on currents initially occur since the incandescent lamp resistance is initially low and only increases as the operating duration increases. The high switch-on currents in conjunction with the low area-specific on resistances bring about high power loss densities and, as a result, large temperature swings in the transistor.
Modern electronic switches generally have a short-circuit protection that causes the lamp to be switched off at excessively high currents. Particularly in the case of short-circuit concepts with temperature measurement, but also in the case of such concepts with current measurement, during the lamp switch-on process in a short time it is then possible for the switch-off to occur again owing to relatively large temperature swings on account of the large switch-on currents of the lamps. The switched-off state persists until the switch has cooled down again sufficiently. A renewed switch-on is thereupon effected with subsequent temperature-dictated (or current-dictated) switch-off. Consequently, the lamp is continuously switched on and off, such that it cannot be permanently switched on and hence cannot be operated properly. Consequently, the maximum switchable loads are reduced.
One procedure that has been practiced heretofore for avoiding this so-called “switch-on cycling” is based on the fact that the switching power loss can be decreased by increasing the switching speed. However, this concept is disadvantageous in practice in particular with regard to the electromagnetic emission and the associated EMC problem (EMC=electromagnetic compatibility).